The findings have caused alarm among scientists, who say the proximity of the 2016 and 2017 bleaching events is unprecedented for the reef, and will give damaged coral little chance to recover.

Scientists with the Australian Research Council’s Centre of Excellence for Coral Reef Studies last week completed aerial surveys of the world’s largest living structure, scoring bleaching at 800 individual coral reefs across 8,000km.

The results show the two consecutive mass bleaching events have affected a 1,500km stretch, leaving only the reef’s southern third unscathed…

Mass bleaching – a phenomenon caused by global warming-induced rises to sea surface temperatures – has occurred on the reef four times in recorded history.

Prof Terry Hughes, who led the surveys, said the length of time coral needed to recover – about 10 years for fast-growing types – raised serious concerns about the increasing frequency of mass bleaching events.

Last year, in the worst-affected areas to the reef’s north, roughly two-thirds of shallow-water corals were lost.

Hughes has warned Australia now faces a closing window to save the reef by taking decisive action on climate change.

The 2017 bleaching is likely to be compounded by other stresses on the reef, including the destructive crown-of-thorns starfish and poor water quality. The category-four tropical cyclone Debbie came too late and too far south for its cooling effect to alleviate bleaching.